Great point, Doc. Mine for now is the stock input tube plus the Tung-Sol 5998 fed from my pc to Khadas Tone Board and using this set up my most favorite pair of cans among all the ones I've tested (TH-X00, HD 6XX, HD 650, Fostex T60RP, Argon t50rp MK3, DT 1990 Pro, and more) has got go be the DT 1990 Pro. Nothing comes even close to them.